High Stiffness Hub Assembly for Proprotor Systems

ABSTRACT

A high stiffness hub assembly for a proprotor system operable to rotate with a mast of a tiltrotor aircraft having helicopter and airplane flight modes. The hub assembly includes a yoke and a constant velocity joint assembly. The yoke has four blade arms each adapted to hold a proprotor blade. The constant velocity joint assembly provides a torque path from the mast to the yoke that includes a trunnion assembly, four drive links and four pillow blocks. The trunnion assembly is coupled to the mast and has four outwardly extending trunnions. Each drive link has a leading bearing coupled to one of the trunnions and a trailing bearing coupled to one of the pillow blocks. Each pillow block is independently mounted between an upper surface of the yoke and a hub plate with two connection members that extend through the yoke and the pillow block.

TECHNICAL FIELD OF THE DISCLOSURE

The present disclosure relates, in general, to proprotor systemsoperable for use on tiltrotor aircraft having helicopter and airplaneflight modes and, in particular, to a high stiffness hub assembly forproprotor systems producing a first-in-plane frequency greater than2.0/rev in airplane flight mode.

BACKGROUND

Tiltrotor aircraft typically include multiple propulsion assemblies thatare positioned near outboard ends of a fixed wing. Each propulsionassembly may include an engine and transmission that provide torque androtational energy to a drive shaft that rotates a proprotor systemincluding a hub assembly and a plurality of proprotor blades. Typically,at least a portion of each propulsion assembly is rotatable relative tothe fixed wing such that the proprotor blades have a generallyhorizontal plane of rotation providing vertical thrust for takeoff,hovering and landing, much like a conventional helicopter, and agenerally vertical plane of rotation providing forward thrust forcruising in forward flight with the fixed wing providing lift, much likea conventional propeller driven airplane. In addition, tiltrotoraircraft can be operated in configurations between the helicopter flightmode and the airplane flight mode, which may be referred to asconversion flight mode.

Physical structures have natural frequencies of vibration that can beexcited by forces applied thereto as a result of operating parametersand/or environmental conditions. These frequencies are determined, atleast in part, by the materials and geometrical dimensions of thestructures. In the case of tiltrotor aircraft, certain structures havingcritical natural frequencies include the fuselage, the fixed wing andvarious elements of the propulsion assemblies. One important operatingparameter of a tiltrotor aircraft is the angular velocity or revolutionsper minute (RPM) of the proprotor blades, which may generate excitationfrequencies corresponding to 1/rev (1 per revolution), 2/rev, 3/rev,etc. An important environmental condition experienced by tiltrotoraircraft is forward airspeed, which may induce proprotor aeroelasticinstability, similar to propeller whirl flutter, which may couple to thefixed wing of tiltrotor aircraft. It has been found that forwardairspeed induced proprotor aeroelastic instability is a limiting factorrelating to the maximum airspeed of tiltrotor aircraft in the airplaneflight mode.

SUMMARY

In a first aspect, the present disclosure is directed to a highstiffness hub assembly for a proprotor system operable to rotate with amast of a tiltrotor aircraft having a helicopter flight mode and anairplane flight mode. The hub assembly includes a yoke and a constantvelocity joint assembly. The yoke has four blade arms each adapted tohold a proprotor blade. The yoke has a rotational plane and an uppersurface. The constant velocity joint assembly provides a torque pathfrom the mast to the yoke. The torque path includes a trunnion assembly;four drive links and four pillow blocks. The trunnion assembly iscoupled to the mast and has four outwardly extending trunnions. Each ofthe drive links has a leading bearing coupled to one of the trunnionsand a trailing bearing coupled to one of the pillow blocks. Each pillowblock is independently mounted between the upper surface of the yoke anda hub plate with two connection members extending through the yoke andthe pillow block. The constant velocity joint assembly provides agimballing degree of freedom for the yoke relative to the mast.

In some embodiments, the hub assembly may include upper and lower hubsprings wherein the upper hub spring is disposed between the trunnionassembly and the hub plate and the lower hub spring is disposed betweenthe trunnion assembly and the yoke such that the upper and lower hubsprings are operable to dampen movement of the yoke in the gimballingdegree of freedom. In certain embodiments, the upper and lower hubsprings may each include a convex spherical outer surface, a concaveinner spherical surface and a series of elastomeric layers separated byinelastic shims. For example, the upper and lower hub springs may behigh performance, ultra stiff springs having between fifteen andtwenty-five inelastic shims. In some embodiments, the constant velocityjoint assembly may be positioned above the rotational plane of the yoke.

In certain embodiments, the trunnion assembly may be a torque splitterenabling the yoke to have a scissoring degree of freedom. In suchembodiments, the torque splitter may include a spline assembly, an uppertrunnion member and a lower trunnion member. The spline assembly may beconfigured to receive the mast through a central opening. The splineassembly may also have a first plurality of outer splines oriented in afirst direction and a second plurality of outer splines oriented in asecond direction that is different from the first direction. The uppertrunnion member may be disposed about the first plurality of outersplines and the lower trunnion member may be disposed about the secondplurality of outer splines. In some embodiments, the first plurality ofouter splines may be arranged helically about the spline assembly in thefirst direction and the second plurality of outer splines may bearranged helically about the spline assembly in the second direction. Incertain embodiments, the spline assembly may include a first outerspline member having the first plurality of outer splines and a secondouter spline member having the second plurality of outer splines.

In some embodiments, the upper and lower trunnion members may beoperable to translate and rotate relative to the spline assembly suchthat translation of the upper and lower trunnion members in a firsttranslational direction relative to the spline assembly causes the uppertrunnion member to rotate relative to the spline assembly in a firstrotational direction and causes the lower trunnion member to rotaterelative to the spline assembly in a second rotational direction that isopposite of the first rotational direction. Similarly, translation ofthe upper and lower trunnion members in a second translationaldirection, that is opposite of the first translational direction,relative to the spline assembly causes the upper trunnion member torotate relative to the spline assembly in the second rotationaldirection and causes the lower trunnion member to rotate relative to thespline assembly in the first rotational direction.

In certain embodiment, each of the leading and trailing bearings mayinclude a series of elastomeric layers separated by inelastic shims withthe elastomeric layers and the inelastic shims formed as sphericalsections having a common focal point. In some embodiments, each of theleading and trailing bearings may be a divided bearing. In someembodiments, the four pillow blocks may be disposed about the yoke atapproximately ninety degree intervals. In certain embodiments, the fourpillow blocks may include first and second pillow blocks that areoppositely disposed on the yoke and generally parallel to each other todefine a first pillow block axis extending generally perpendicularlytherebetween. In addition, the four pillow blocks may include third andfourth pillow blocks that are oppositely disposed on the yoke andgenerally parallel to each other to define a second pillow block axisextending generally perpendicular therebetween that is generallyorthogonal to the first pillow block axis.

In some embodiments, each pillow block may include upper and lower beamseach having first and second ends with the upper and lower beamsextending generally parallel to each other. Each pillow block may alsoinclude a first arm that extends between the first ends of the upper andlower beams that has a distal end with a hole and a second arm thatextends between the second ends of the upper and lower beams that has adistal end with a hole. The first and second arms are generally parallelto each other and form angles relative to the upper and lower beams. Acoupling element may extend through the holes generally perpendicularlyto the first and second arms. The coupling element may receive thetrailing end of one of the drive links. The upper and lower beams andthe first arm may define a generally perpendicularly extending firstopening and the upper and lower beams and the second arm may define agenerally perpendicularly extending second opening through which the twoconnection members couple the pillow block between the upper surface ofthe yoke and the hub plate.

In a second aspect, the present disclosure is directed to a proprotorsystem for tiltrotor aircraft having a helicopter flight mode and anairplane flight mode. The proprotor system includes a mast, a yokehaving four blade arms and an upper surface and a plurality of proprotorblades each coupled to one of the blade arms of the yoke. A constantvelocity joint assembly provides a torque path from the mast to theyoke. The torque path includes a trunnion assembly, four drive links andfour pillow blocks. The trunnion assembly is coupled to the mast and hasfour outwardly extending trunnions. Each of the drive links has aleading bearing coupled to one of the trunnions and a trailing bearingcoupled to one of the pillow blocks. Each pillow block is independentlymounted between the upper surface of the yoke and a hub plate with twoconnection members extending through the yoke and the pillow block. Theconstant velocity joint assembly provides a gimballing degree of freedomfor the yoke relative to the mast. An upper hub spring is disposedbetween the trunnion assembly and the hub plate and a lower hub springis disposed between the trunnion assembly and the yoke. The upper andlower hub springs are operable to dampen movement of the yoke in thegimballing degree of freedom. In the airplane flight mode, a firstin-plane frequency of each proprotor blade is greater than 2.0/rev.

In some embodiments, in the airplane flight mode, the first in-planefrequency of each proprotor blade is less than 3.0/rev. In certainembodiments, in the airplane flight mode, the first in-plane frequencyof each proprotor blade is between about 2.2/rev and about 2.8/rev. Inother embodiments, in the airplane flight mode, the first in-planefrequency of each proprotor blade is between about 2.4/rev and about2.6/rev. In some embodiments, the proprotor system may have a leadingedge pitch horn and a pitch control assembly having a positive delta 3angle coupled to each proprotor blade.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features and advantages of thepresent disclosure, reference is now made to the detailed descriptionalong with the accompanying figures in which corresponding numerals inthe different figures refer to corresponding parts and in which:

FIGS. 1A-1B are schematic illustrations of a tiltrotor aircraft in anairplane flight mode and a helicopter flight mode, respectively, inaccordance with embodiments of the present disclosure;

FIG. 2A-2F are various views of a proprotor system for use on atiltrotor aircraft in accordance with embodiments of the presentdisclosure;

FIG. 3 is an exploded view of a constant velocity joint assembly of aproprotor system for use on a tiltrotor aircraft in accordance withembodiments of the present disclosure;

FIGS. 4A-4B are cross sectional views of upper and lower hub springs ofa proprotor system for use on a tiltrotor aircraft in accordance withembodiments of the present disclosure;

FIG. 5 is an exploded view of a torque splitter of a proprotor systemfor use on a tiltrotor aircraft in accordance with embodiments of thepresent disclosure;

FIGS. 6A-6B are cross sectional views of a drive link of a proprotorsystem for use on a tiltrotor aircraft in accordance with embodiments ofthe present disclosure; and

FIG. 7 is an exploded view of pillow block connections of a proprotorsystem for use on a tiltrotor aircraft in accordance with embodiments ofthe present disclosure.

DETAILED DESCRIPTION

While the making and using of various embodiments of the presentdisclosure are discussed in detail below, it should be appreciated thatthe present disclosure provides many applicable inventive concepts,which can be embodied in a wide variety of specific contexts. Thespecific embodiments discussed herein are merely illustrative and do notdelimit the scope of the present disclosure. In the interest of clarity,not all features of an actual implementation may be described in thepresent disclosure. It will of course be appreciated that in thedevelopment of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedeveloper's specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming but would be a routine undertakingfor those of ordinary skill in the art having the benefit of thisdisclosure.

In the specification, reference may be made to the spatial relationshipsbetween various components and to the spatial orientation of variousaspects of components as the devices are depicted in the attacheddrawings. However, as will be recognized by those skilled in the artafter a complete reading of the present disclosure, the devices,members, apparatuses, and the like described herein may be positioned inany desired orientation. Thus, the use of terms such as “above,”“below,” “upper,” “lower” or other like terms to describe a spatialrelationship between various components or to describe the spatialorientation of aspects of such components should be understood todescribe a relative relationship between the components or a spatialorientation of aspects of such components, respectively, as the devicedescribed herein may be oriented in any desired direction.

Referring to FIGS. 1A and 1B in the drawings, a tiltrotor aircraft isschematically illustrated and generally designated 10. Aircraft 10includes a fuselage 12, a wing mount assembly 14 that is rotatablerelative to fuselage 12 and a tail assembly 16 having control surfacesoperable for horizontal and/or vertical stabilization during forwardflight. A wing 18 is supported by wing mount assembly 14 and rotateswith wing mount assembly 14 relative to fuselage 12 to enable tiltrotoraircraft 10 convert to a storage configuration. Together, fuselage 12,tail assembly 16 and wing 18 as well as their various frames, longerons,stringers, bulkheads, spars, ribs, skins and the like may be consideredto be the airframe of tiltrotor aircraft 10.

Located proximate the outboard ends of wing 18 are fixed nacelles 20 a,20 b, each of which preferably houses an engine and a fixed portion of adrive system. A pylon assembly 22 a is rotatable relative to fixednacelle 20 a and wing 18 between a generally horizontal orientation, asbest seen in FIG. 1A, a generally vertical orientation, as best seen inFIG. 1B. Pylon assembly 22 a includes a rotatable portion of the drivesystem and a proprotor system 24 a that is rotatable responsive totorque and rotational energy provided via the engine and drive system.Likewise, a pylon assembly 22 b is rotatable relative to fixed nacelle20 b and wing 18 between a generally vertical orientation, as best seenin FIG. 1A, a generally horizontal orientation, as best seen in FIG. 1B.Pylon assembly 22 b includes a rotatable portion of the drive system anda proprotor system 24 b that is rotatable responsive to torque androtational energy provided via the engine and drive system. In theillustrated embodiment, proprotor systems 24 a, 24 b each include fourproprotor blades 26. It should be understood by those having ordinaryskill in the art, however, that proprotor assemblies 24 a, 24 b couldalternatively have a different number of proprotor blades, either lessthan or greater than four. In addition, it should be understood that theposition of pylon assemblies 22 a, 22 b, the angular velocity orrevolutions per minute (RPM) of the proprotor systems 24 a, 24 b, thepitch of proprotor blades 26 and the like are controlled by the pilot oftiltrotor aircraft 10 and/or the flight control system to selectivelycontrol the direction, thrust and lift of tiltrotor aircraft 10 duringflight.

FIG. 1A illustrates tiltrotor aircraft 10 in a forward flight mode orairplane flight mode, in which proprotor systems 24 a, 24 b arepositioned to rotate in a substantially vertical plane to provide aforward thrust while a lifting force is supplied by wing 18 such thattiltrotor aircraft 10 flies much like a conventional propeller drivenaircraft. FIG. 1B illustrates tiltrotor aircraft 10 in a verticaltakeoff and landing (VTOL) flight mode or helicopter flight mode, inwhich proprotor systems 24 a, 24 b are positioned to rotate in asubstantially horizontal plane to provide a vertical thrust such thattiltrotor aircraft 10 flies much like a conventional helicopter. Duringoperation, tiltrotor aircraft 10 may convert from helicopter flight modeto airplane flight mode following vertical takeoff and may convert backto helicopter flight mode from airplane flight mode for hover andvertical landing. In addition, tiltrotor aircraft 10 can perform certainflight maneuvers with proprotor systems 24 a, 24 b positioned betweenairplane flight mode and helicopter flight mode, which can be referredto as conversion flight mode.

Preferably, each fixed nacelle 20 a, 20 b houses a drive system, such asan engine and transmission, for supplying torque and rotational energyto a respective proprotor system 24 a, 24 b. In such embodiments, thedrive systems of each fixed nacelle 20 a, 20 b may be coupled togethervia one or more drive shafts located in wing 18 such that either drivesystem can serve as a backup to the other drive system in the event of afailure. Alternatively or additionally, fuselage 12 may include a drivesystem, such as an engine and transmission, for providing torque androtational energy to each proprotor system 24 a, 24 b via one or moredrive shafts located in wing 18. In tiltrotor aircraft having bothnacelle and fuselage mounted drive systems, the fuselage mounted drivesystem may serve as a backup drive system in the event of failure ofeither or both of the nacelle mounted drive systems.

In general, proprotor systems for tiltrotor aircraft should be designedto achieve blade flap or out-of-plane frequencies and lead-lag orin-plane frequencies that are sufficiently distant from the excitationfrequencies generated by the proprotor systems corresponding to 1/rev (1per revolution), 2/rev, 3/rev, etc. As an example, if a proprotor systemhas an operating speed of 360 RPM, the corresponding 1/rev excitationfrequency is 6 Hertz (360/60=6 Hz). Similarly, the corresponding 2/revexcitation frequency is 12 Hz and the corresponding 3/rev excitationfrequency is 18 Hz. It should be understood by those having ordinaryskill in the art that a change in the operating speed of a proprotorsystem will result in a proportional change in the excitationfrequencies generated by the proprotor system. For tiltrotor aircraft,operating in airplane flight mode typically requires less thrust thanoperating in helicopter flight mode. One way to reduce thrust as well asincrease endurance, reduce noise levels and reduce fuel consumption isto reduce the operating speed of the proprotor systems. For example, inhelicopter flight mode, the tiltrotor aircraft may operate at 100percent of design RPM, but in airplane flight mode, the tiltrotoraircraft may operate at a reduced percent of design RPM such as betweenabout 80 percent and about 90 percent of design RPM. Thus, to achievedesirable rotor dynamics, the proprotor systems for tiltrotor aircraftshould be designed to avoid the frequencies of 1/rev, 2/rev, 3/rev, etc.for both helicopter flight mode and airplane flight mode operations.

To achieve acceptable rotor dynamics, conventional tiltrotor aircrafthave operated proprotor systems having three twisted proprotor bladesutilizing negative 15 degrees delta 3 pitch-flap coupling and having afirst-in-plane frequency in airplane flight mode of about 1.4/rev. Delta3 refers to the angle measured about the rotational axis of theproprotor system from an axis normal to the pitch change axis to thepitch horn attachment point of a proprotor blade. Delta 3 pitch-flapcoupling is used to reduce or control the degree of blade flapping byautomatically changing the blade pitch as the blade flaps up or downrelative to its flap axis. It is noted that to achieve desiredsatiability for a conventional helicopter, when a blade raises about itsflap axis, the blade pitch is reduced by the delta 3 pitch-flapcoupling, which is known as positive delta 3 (flap up/pitch down). Toachieve desired satiability for a conventional tiltrotor aircraft,however, when a blade raises about its flap axis, the blade pitch isincreased by the delta 3 pitch-flap coupling, which is known as negativedelta 3 (flap up/pitch up).

During high speed operations in airplane flight mode, it is important tocontrol proprotor blade flapping on a tiltrotor aircraft, as the forwardairspeed may induce proprotor aeroelastic instability, similar topropeller whirl flutter, that may couple to the wing and lead tofailures. In addition, it can be important to maintain the flappingfrequency sufficiently distant from the first-in-plane frequency. Toachieve this balance, conventional tiltrotor aircraft have utilized anegative delta 3 angle of 15 degrees. Due to the location requirementsfor the pitch links and pitch horns necessary to achieve the negative 15degrees delta 3 configuration, proprotor systems have been limited tothe conventional three blade design. It is noted that for reasonsincluding pilot fatigue, passenger comfort, noise reduction andvibration induced mechanical failures, to name a few, it is desirable tohave more than three proprotor blades on each proprotor system of atiltrotor aircraft.

In the illustrated embodiment, each proprotor system 24 a, 24 b includesfour proprotor blades 26 that are positioned circumferentially about ahub assembly at ninety degree intervals. Proprotor blades 26 and the hubassembly are preferably designed to have sufficient stiffness to shiftthe first-in-plane frequency of proprotor blades 26, when tiltrotoraircraft 10 is in airplane flight mode, from the conventional 1.4/rev toa first-in-plane frequency above 2.0/rev. For example, the firstin-plane frequency of proprotor blades 26 may be in a range betweenabout 2.0/rev and about 3.0/rev. In some embodiments, the first in-planefrequency of proprotor blades 26 may preferably be in a range betweenabout 2.2/rev and about 2.8/rev and more preferably in a range betweenabout 2.4/rev and about 2.6/rev. Moving the first-in-plane frequencyabove 2.0/rev decouples the first-in-plane lead-lag frequency from theout-of-plane flapping frequency. This decoupling enables a shift fromthe conventional negative 15 degrees delta 3 configuration to a positivedelta 3 configuration including up to about a positive 30 degrees delta3 configuration. Using the disclosed positive delta 3 configuration, aswell as leading edge pitch links and pitch horns, no longer limit theproprotor design to the conventional three blade configuration andenable the four blade configurations of the embodiments herein.

The desired proprotor blade stiffness and/or stiffness to mass ratio ofthe present embodiments is achieved using, for example, carbon-basedmaterials for the structural components of proprotor blades 26 such asgraphite-based materials, graphene-based materials or other carbonallotropes including carbon nanostructure-based materials such asmaterials including single-walled and multi-walled carbon nanotubes. Inone example, the spar and/or skin of proprotor blades 26 are preferablymonolithic structures formed using a broad goods and/or layered tapeconstruction process having a manual or automated layup of a pluralityof composite broad goods material layers including carbon fabrics,carbon tapes and combinations thereof, positioned over one or moremandrels having simple geometric surfaces with smooth transitions. Aftercuring and other processing steps, the material layers form a highstrength, lightweight solid composite members. In this process, thematerial thicknesses of the components can be tailoring spanwise andchordwise to the desired stiffness and/or stiffness to mass ratio. Theproprotor blade components may be composed of up to about 50 percent,about 60 percent, about 70 percent, about 80 percent, about 90 percentor more of the carbon-based material or materials.

Referring next to FIGS. 2A-2F in the drawings, a proprotor system fortiltrotor aircraft is depicted and generally designated 100. In theillustrated embodiment, proprotor system 100 includes a hub assembly 102including a yoke 104 that is coupled to a mast 106 via a constantvelocity joint assembly 108. Hub assembly 102 rotates with mast 106,which is coupled to a drive system including an engine and transmissionof the tiltrotor aircraft that provides torque and rotational energy toproprotor system 100. As discussed herein, constant velocity jointassembly 108 provides a gimballing degree of freedom for yoke 104relative to mast 106 enabling yoke 104 to teeter in any directionrelative to the rotational axis 110 of proprotor system 100.Accordingly, hub assembly 102 may be referred to as a gimbaled hub. Inthe illustrated implementation, constant velocity joint assembly 108 ispositioned above the rotational plane of yoke 104 and is mounted onand/or coupled to an upper surface of yoke 104. As illustrated, yoke 104includes four blade arms each of which holds and supports a proprotorblade 112. Each proprotor blade 112 includes a spar 114 that extendsspanwise toward the tip of proprotor blade 112. Spars 114 are preferablythe main structural members of proprotor blades 112 designed to carrythe primary centrifugal and bending loads of proprotor blades 112. Spars114 may have a root-to-tip twist on the order of about 30 degrees toabout 40 degrees or other suitable root-to-tip twist.

Each spar 114 has a root section depicted as integral cuff 116 to enablecoupling of each proprotor blade 112 with hub assembly 102 via bearingassemblies 118, 120. Each bearing assembly 118 is coupled to yoke 104with a plurality of connecting members such as bolts, pins or the like.Likewise, each bearing assembly 120 is coupled to yoke 104 with aplurality of connecting members such as bolts, pins or the like. Eachbearing assembly 120 includes a rotatably mounted beam assembly 122having upper and lower blade grips 122 a, 122 b. As illustrated, eachspar 114 is coupled to a respective beam assembly 122 at upper and lowerblade grips 122 a, 122 b with a plurality of connecting members such asbolts, pins or the like. In addition, each spar 114 is coupled to arespective bearing assembly 118 via a suitable connection (not visible).Each spar 114 has a centrifugal force retention load path throughintegral cuff 116 via bearing assemblies 118, 120 to yoke 104. In theillustrated embodiment, each spar 114 includes an integral pitch horn124 on the leading edge of spar 114 that is coupled to a leading edgepitch link 126 of a pitch control assembly 128 depicted as the rotatingportion of a rise and fall swash plate operable to collectively andcyclically control the pitch of proprotor blades 112. Each proprotorblade 112 is operable to independently rotate relative to hub assembly102 about its pitch change axis and thereby change pitch responsive tochanges in position of the respective pitch link 126. Rotation of eachproprotor blade 112 causes the respective beam assembly 122 to rotaterelative to yoke 104 about the pitch change axis.

Preferably, spar 114 and the skin of each proprotor blade 112 are formedfrom a carbon-based material such that the proprotor blade stiffnessand/or stiffness to mass ratio is sufficient to enable the first-in-planfrequency of proprotor blades 112 in the airplane flight mode of thetiltrotor aircraft to be in a range between about 2.0/rev and about3.0/rev, more preferably in a range between about 2.2/rev and about2.8/rev and most preferably in a range between about 2.4/rev and about2.6/rev. By establishing the first-in-plane frequency above 2.0/rev andthus decoupling the first-in-plane lead-lag frequency from theout-of-plane flapping frequency, the delta 3 angle can be shifted fromthe conventional negative 15 degrees delta 3 configuration to a positivedelta 3 configuration including up to about a positive delta 3 angle of30 degrees. As best seen in FIG. 2A, angle 130 represents the positivedelta 3 configuration of the present embodiment, wherein the delta 3angle is about positive 30 degrees. Implementing the illustratedpositive delta 3 configuration enables the four blade design ofproprotor system 100 while avoiding interference between pitch links 126and other components of proprotor system 100. As best seen in FIG. 2F,spar 114 and skin 132 of each proprotor blade 112 are preferably formedfrom a carbon-based material. In addition, each proprotor blade 112 mayinclude a core 134 that provides stability, compression resistance andshear transfer between the upper and lower portions of skin 132. Core134 may be formed from a carbon-based material, a nomex honeycombstructure or other suitable material.

Referring next to FIG. 3 of the drawings, constant velocity jointassembly 108 is described in greater detail. Constant velocity jointassembly 108 is provided between yoke 104 and mast 106 to allow mast 106to transmit power through a variable angle, at constant speed, withoutan appreciable increase in friction or play, thereby providing thegimballing degree of freedom for yoke 104 relative to mast 106. Ingeneral, a constant velocity (CV) joint may refer to a type of mechanismthat connects two rotating components making an angle with one another.This angle may vary during service, such as may be the case with theangle between yoke 104 and mast 106. A CV joint may mechanically couplean input element to an output element in such a way that torque may betransmitted from the input element to the output element whilemaintaining a substantially CV characteristic. A CV characteristicrefers to a characteristic wherein the instantaneous angular velocity ofthe input element is substantially matched to the instantaneous angularvelocity of the output element throughout a full rotation of theelement. It is to be understood that the CV characteristic may representa design goal, and various embodiments may achieve this characteristicto a greater or lesser degree based on parameters, which may includemechanical and structural variations in the assembly. Thus, a joint maymaintain a substantially CV characteristic even if the angularvelocities do not perfectly match. In some embodiments, a CV joint maymaintain a substantially CV characteristic despite variations in anglebetween the input and output element.

In the illustrated embodiment, constant velocity joint assembly 108includes a cap 200, a hub plate 202, an upper hub spring 204, an upperspherical adaptor 206, a trunnion assembly 208, four drive links 210,four pillow blocks 212, a lower spherical adaptor 214, a lower hubspring 216 and a lower hub spring support 218. Hub plate 202 includeseight hub plates arms 220 that extend radially outwardly from a hubplate body 222 at approximately forty-five degree intervals. In theillustrated embodiment, each hub plate arm 220 includes an opening (notvisible) having a bushing 224 disposed therein that is sized to extendinto an upper portion of an opening 250 of a respective pillow block212. Alternatively, hub plate body 222 and pillow blocks 212 may haveindependent bushings. Internally, hub plate body 222 has a concave innerspherical surface 226 that is operable to receive and preferably beadhered to a convex outer spherical surface 228 of upper hub spring 204.Upper hub spring 204 includes a concave inner spherical surface (notvisible) that is operable to receive and preferably be adhered to aconvex outer spherical surface 230 of upper spherical adaptor 206.

Upper spherical adaptor 206 may be adhered or coupled to an uppersurface 232 of trunnion assembly 208. Alternatively, upper sphericaladaptor 206 may be integral with trunnion assembly 208. In theillustrated embodiment, trunnion assembly 208 is received on mast 106via a matching spline coupling. Trunnion assembly 208 includes fouroutwardly extending trunnions 234. Two of the trunnions 234 areoppositely disposed on an upper trunnion member 236 and two of thetrunnions 234 are oppositely disposed on a lower trunnion member 238. Asdiscussed herein, upper trunnion member 236 and lower trunnion member238 are operable to counter rotate relative to one another to provide ascissoring degree of freedom to yoke 104. Each trunnion 234 is receivedwithin an opening 240 of a leading bearing 242 of one of the drive links210. An opening 244 of a trailing bearing 246 of each drive link 210 isreceived on a coupling element 248 of one of the pillow blocks 212. Eachpillow block has a pair of openings 250 extending therethrough. An upperportion of each opening 250 is operable to receive a bushing 224therein.

Lower spherical adaptor 214 may be adhered or coupled to a lower surface(not visible) of trunnion assembly 208. Alternatively, lower sphericaladaptor 214 may be integral with trunnion assembly 208. Lower sphericaladaptor 214 includes a convex outer spherical surface 252 that isoperable to be received within and preferably be adhered to a concaveinner spherical surface 254 of lower hub spring 216. Lower hub spring216 has a convex outer spherical surface 256 that is operable to bereceived within and preferably be adhered to a concave inner sphericalsurface 258 of lower hub spring support 218. Lower hub spring support218 is received within a central opening 260 of yoke 104. In theillustrated embodiment, yoke 104 includes eight openings (not visible)each having a bushing 262 disposed therein that is sized to extend intoa lower portion of each opening 250 a respective pillow block 212.Alternatively, yoke 104 and pillow blocks 212 may have independentbushings.

Constant velocity joint assembly 108 is mounted on the upper surface 264of yoke 104 using suitable connection member such as eight bolts thatextend through the eight openings of yoke 104, the four pairs of twoopenings 250 of pillow blocks 212 and the eight openings of hub plate202. The torque path from mast 106 to yoke 104 is as follows: torque istransferred from mast 106 to trunnion assembly 208 via the splinedcoupling therebetween. Torque then transfers to the leading bearings 242of drive links 210 via trunnion 234. It is then transferred from thetrailing bearings 246 of drive links 210 to pillow blocks 212 viacoupling elements 248. Torque is then transferred to yoke 104 frompillow blocks 212 via the connection member.

Referring additionally to FIGS. 4A-4B in the drawings, therein aredepicted cross sectional views of upper hub spring 204 and lower hubspring 216, respectively. In the illustrated embodiment, upper hubspring 204 and lower hub spring 216 each comprises a laminated sphericalcomponent that includes a series of layers of elastomeric material ofpredetermined thickness separated by a series of relatively inelastic,or non-extensible, members, such as metal shims between the elastomericlayers. Upper hub spring 204 and lower hub spring 216 are preferablyhigh performance, ultra stiff hub springs the utilize a high number ofmetal shims, such as between fifteen and twenty-five metal shims, withnineteen metal shims depicted in the drawings. The outermost elastomericlayer of upper hub spring 204 has a spherical convex surface 228 and theinnermost elastomeric layer has a spherical concave surface 270. Each ofthe elastomeric layers and metal shims of upper hub spring 204 is formedas a spherical section and is positioned so that their focal points arecoincident. Likewise, the outermost elastomeric layer of lower hubspring 216 has a spherical convex surface 272 and the innermostelastomeric layer has a spherical concave surface 254. Each of theelastomeric layers and metal shims of lower hub spring 216 is formed asa spherical section and is positioned so that their focal points arecoincident. In the operational configuration of constant velocity jointassembly 108, the focal points of the elastomeric layers and metal shimsof upper hub spring 204 are preferably coincident with the focal pointof the elastomeric layers and metal shims of lower hub spring 216 suchthat upper hub spring 204 and lower hub spring 216 are components of asingle spherical element. Upper hub spring 204 and lower hub spring 216have spring rates that counteract movement of yoke 104 in its gimballingdegree of freedom.

Referring additionally to FIG. 5 in the drawings, therein is depicted anexploded view of a torque splitter 300 disposed within trunnion assembly208. Torque splitter 300 includes an inner spline member 302 depicted asinner spline element 302 a and inner spline element 302 b. Inner splineelements 302 a, 302 b are preferably coupled together or may be integralwith each other and include an inner spline that allows inner splinemember 302 to receive torque from mast 106. Inner spline member 302includes an outer spline that allows outer spline members 304 a, 304 bto move axially relative to inner spline member 302. Outer splinemembers 304 a, 304 b include inner splines that correspond to the outersplines of inner spline member 302. In some embodiments, outer splinemembers 304 a, 304 b may be coupled together such that outer splinemembers 304 a, 304 b move together axially relative to inner splinemember 302.

Outer spline members 304 a, 304 b include outer splines oriented indifferent directions. For example, outer spline member 304 a has helicalsplines oriented at a first direction while outer spline member 304 bhas helical splines oriented at a second direction different from andpreferably opposite of the first direction. Upper trunnion member 236includes inner splines that correspond to the outer splines of outerspline member 304 a. Lower trunnion member 238 includes inner splinesthat correspond to the outer splines of outer spline member 304 b.Torque splitter 300 may also include a variety of bearing members, suchas upper and lower trunnion bearings 306 a, 306 b and spline bearings308 a, 308 b. As illustrated, upper trunnion bearing 306 a separatesupper trunnion member 236 from outer spline member 304 a, lower trunnionbearing 306 b separates lower trunnion member 238 from outer splinemember 304 b, spline bearing 308 a separates outer spline member 304 afrom inner spline members 302 a and spline bearing 308 b separates outerspline member 304 b from inner spline members 302 b. In some exampleembodiments, upper and lower trunnion bearings 306 a, 306 b and splinebearings 308 a, 308 b may be made from an elastomeric material, such asrubber. A spacer 310 may be provided that separates upper and lowertrunnion members 236, 238. Upper and lower retention members 312 a, 312b, upper and lower retention bearings 314 a, 314 b and a cap 316,separately or in combination, prevent certain axial displacement ofupper and lower trunnion members 236, 238 relative to mast 106.

In this configuration, spline assembly 302 allows upper and lowertrunnion members 236, 238 to transmit equal drive torque while movingrelative to each other in a kinematic scissoring motion, which mayeliminate some kinematic binding forces. For example, upper and lowertrunnion members 236, 238 may scissor relative to each other in responseto the scissoring degree of freedom of yoke 104. In some embodiments,torque splitter 300 allows upper and lower trunnion members 236, 238 toscissor relative to each other by allowing outer spline members 304 a,304 b to move axially in a direction that allows upper and lowertrunnion members 236, 238 to scissor on opposing helical splines. Forexample, when one trunnion member begins to lead or lag, an unbalancedaxial load in the helical splines causes displacement of outer splinemembers 304 a, 304 b relative to inner spline member 302 such thatequilibrium is restored and the torque is balanced. It should be notedthat while upper and lower retention members 312 a, 312 b prevent axialmovement of upper and lower trunnion members 236, 238, outer splinemembers 304 a, 304 b may be free to move axially without being limitedby upper and lower retention members 312 a, 312 b.

Referring additionally to FIGS. 6A-6B in the drawings, therein aredepicted cross sectional views of a drive link 210 including a leadingbearing 242 and a trailing bearing 246. Leading bearing 242 includes acentral bushing 320 having an opening 240. Training bearing 246 includesa central bushing 322 having an opening 244. As discussed herein,opening 240 of leading bearing 242 receives a trunnion 234 therein andopening 244 of trailing bearing 246 receives a coupling element 248 of apillow blocks 212 therein. Leading and trailing bearings 242, 246 areeach formed from a series of layers of elastomeric material ofpredetermined thickness separated by a series of relatively inelastic,or non-extensible, members, such as metal shims between the elastomericlayers. The outermost elastomeric layer of each bearing is disposedadjacent to a spherical concave surface of the body of drive link 210,as best seen in FIG. 6B. Likewise, the innermost elastomeric layer isdisposed adjacent to a convex spherical surface of the respectivecentral bushing 320, 322, as best seen in FIG. 6B. The elastomericlayers and metal shims in leading bearing 242 are each sphericalsections shaped so that their focal points are coincident at location324. The elastomeric layers and metal shims in trailing bearing 246 areeach spherical sections shaped so that their focal points are coincidentat location 326. Leading and trailing bearings 242, 246 are preferablyhigh performance, ultra stiff bearings the utilize a high number ofmetal shims, such as between fifteen and twenty-five metal shims, withsixteen metal shims depicted in the drawings.

During operation of the proprotor system using drive links 210, astorque is transmitted from trunnion assembly 208 to pillow blocks 212,the elastomeric layers at the leading edge of leading bearing 242 andthe trailing edge of trailing bearing 246 are subjected substantiallyentirely to cyclically varying compressive loads while thediametrically-opposite elastomeric layers of leading bearing 242 andtrailing bearing 246 may be subjected substantially entirely tocyclically varying tensile loads. To minimize the cyclically varyingtensile loads, which may cause molecular tearing, cavitation, acceleratepropagation of fatigue cracking and/or elastomer degradation, leadingbearing 242 and trailing bearing 246 are preferably divided bearingswherein the elastomeric layers and metal shims are discontinuous aboutthe periphery of the bearing. In the illustrated embodiment, the metalshims include inner sections that extend approximately ninety degreesabout each bearing and outer sections that extend approximately twohundred and sixty degrees about each bearing. In addition, there are twovoids that separate inner and outer sections of the elastomeric layersthat are approximately ninety degrees apart. By providing suchseparations in each of the bearings, tensile type stresses in theelastomeric layers can be avoided, thereby enhancing the fatigue life ofbearings.

Referring additionally to FIG. 7 in the drawings, therein is depicted asimplified exploded view showing the mating components for couplingconstant velocity joint assembly 108 to yoke 104. The illustratedsection includes hub plate 202, four pillow blocks 212, yoke 104 andeight connection members depicted as bolts 350, nuts 352 and lockwashers 354. As described herein, hub plate 202 includes eight hub platearms 220 that extend radially outwardly from a hub plate body 222 atapproximately forty-five degree intervals. Each hub plate arm 220includes an opening having a bushing 224 disposed therein that is sizedto extend into an upper portion of an opening 250 of a respective pillowblock 212. As illustrated, four independent pillow blocks 212 aredisposed about yoke 104 at approximately ninety-degree intervals. Inthis configuration, there are two set of oppositely disposed pillowblocks 212. The pillow blocks 212 in each set are generally parallel toeach other such that one set defines a pillow block axis 356 extendinggenerally perpendicularly therebetween and the other set defines apillow block axis 358 extending generally perpendicularly therebetweenthat is generally orthogonal to pillow block axis 356.

Each pillow block 212 includes upper and lower beams 360, 362 that aregenerally parallel to each other. Arms 364, 366 extend between upper andlower beams 360, 362. Arms 364, 366 are generally parallel to each otherand extend at angles relative to upper and lower beams 360, 362. Eacharm 364, 366 has a distal end with a hole (not visible). A couplingelement 248 extends through the respective holes of arms 364, 366 and iscoupled thereto. Coupling element 248 extends generally perpendicularlybetween arms 364, 366. Upper and lower beams 360, 362 together with arm364 define one of the generally perpendicularly extending openings 250through pillow block 212. Likewise, upper and lower beams 360, 362together with arm 366 define the other of the generally perpendicularlyextending openings 250 through pillow block 212. An upper portion ofeach opening 250 is operable to receive a bushing 224 therein. A lowerportion of each opening 250 is operable to receive a bushing 262therein. In the illustrated embodiment, yoke 104 includes eight openings(not visible) each having a bushing 262 disposed therein that is sizedto extend into a lower portion of an opening 250 of a respective pillowblock 212. Constant velocity joint assembly 108 is mounted on the uppersurface 264 of yoke 104 and secured thereto with the eight connectionmembers, two of which extend through each pillow block 212. Through theuse of four independent pillow blocks 212 with two connection memberscoupling each between yoke 104 and hub plate 202, a high stiffness hubassembly 108 is formed.

The foregoing description of embodiments of the disclosure has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the disclosure to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from practice of the disclosure. Theembodiments were chosen and described in order to explain the principalsof the disclosure and its practical application to enable one skilled inthe art to utilize the disclosure in various embodiments and withvarious modifications as are suited to the particular use contemplated.Other substitutions, modifications, changes and omissions may be made inthe design, operating conditions and arrangement of the embodimentswithout departing from the scope of the present disclosure. Suchmodifications and combinations of the illustrative embodiments as wellas other embodiments will be apparent to persons skilled in the art uponreference to the description. It is, therefore, intended that theappended claims encompass any such modifications or embodiments.

What is claimed is:
 1. A high stiffness hub assembly for a proprotorsystem operable to rotate with a mast of a tiltrotor aircraft having ahelicopter flight mode and an airplane flight mode, the hub assemblycomprising: a yoke having four blade arms each adapted to hold aproprotor blade, the yoke having a rotational plane and an uppersurface; and a constant velocity joint assembly providing a torque pathfrom the mast to the yoke, the torque path including a trunnionassembly, four drive links and four pillow blocks, the trunnion assemblycoupled to the mast and having four outwardly extending trunnions, eachof the drive links having a leading bearing coupled to one of thetrunnions and a trailing bearing coupled to one of the pillow blocks andeach pillow block independently mounted between the upper surface of theyoke and a hub plate with two connection members extending through theyoke and the pillow block; wherein, the yoke has a gimballing degree offreedom relative to the mast.
 2. The hub assembly as recited in claim 1further comprising upper and lower hub springs, the upper hub springdisposed between the trunnion assembly and the hub plate, the lower hubspring disposed between the trunnion assembly and the yoke, the upperand lower hub springs operable to dampen movement of the yoke in thegimballing degree of freedom.
 3. The hub assembly as recited in claim 2wherein the upper and lower hub springs each further comprises a convexspherical outer surface, a concave inner spherical surface and a seriesof elastomeric layers separated by inelastic shims.
 4. The hub assemblyas recited in claim 3 wherein each series of elastomeric layersseparated by inelastic shims of the upper and lower hub springs furthercomprises between fifteen and twenty-five inelastic shims.
 5. The hubassembly as recited in claim 1 wherein the constant velocity jointassembly is positioned above the rotational plane of the yoke.
 6. Thehub assembly as recited in claim 1 wherein the trunnion assembly furthercomprises a torque splitter enabling the yoke to have a scissoringdegree of freedom.
 7. The hub assembly as recited in claim 6 wherein thetorque splitter further comprises: a spline assembly configured toreceive the mast through a central opening, the spline assembly having afirst plurality of outer splines oriented in a first direction and asecond plurality of outer splines oriented in a second directiondifferent from the first direction; an upper trunnion member disposedabout the first plurality of outer splines; and a lower trunnion memberdisposed about the second plurality of outer splines.
 8. The hubassembly as recited in claim 7 wherein the first plurality of outersplines further comprise first helically arranged outer splines and thesecond plurality of outer splines further comprise second helicallyarranged outer splines.
 9. The hub assembly as recited in claim 7wherein the spline assembly further comprises a first outer splinemember having the first plurality of outer splines and a second outerspline member having the second plurality of outer splines.
 10. The hubassembly as recited in claim 7 wherein the upper and lower trunnionmembers are operable to translate and rotate relative to the splineassembly; wherein, translation of the upper and lower trunnion membersin a first translational direction relative to the spline assemblycauses the upper trunnion member to rotate relative to the splineassembly in a first rotational direction and causes the lower trunnionmember to rotate relative to the spline assembly in a second rotationaldirection that is opposite of the first rotational direction; andwherein, translation of the upper and lower trunnion members in a secondtranslational direction that is opposite of the first translationaldirection relative to the spline assembly causes the upper trunnionmember to rotate relative to the spline assembly in the secondrotational direction and causes the lower trunnion member to rotaterelative to the spline assembly in the first rotational direction. 11.The hub assembly as recited in claim 1 wherein each of the leading andtrailing bearings further comprises a series of elastomeric layersseparated by inelastic shims; and wherein, each of the elastomericlayers and the inelastic shims further comprises a spherical sectionhaving a common focal point.
 12. The hub assembly as recited in claim 11wherein each of the leading and trailing bearings further comprises adivided bearing.
 13. The hub assembly as recited in claim 1 wherein thefour pillow blocks are disposed about the yoke at approximately ninetydegree intervals.
 14. The hub assembly as recited in claim 1 whereineach pillow block further comprises: upper and lower beams each havingfirst and second ends, the upper and lower beams extending generallyparallel to each other; a first arm extending between the first ends ofthe upper and lower beams and having a distal end with a hole; a secondarm extending between the second ends of the upper and lower beams andhaving a distal end with a hole, the first and second arms generallyparallel to each other and forming angles relative to the upper andlower beams; and a coupling element extending generally perpendicular tothe first and second arms and through the holes, the coupling elementreceiving the trailing bearing of one of the drive links; wherein, theupper and lower beams and the first arm define a generallyperpendicularly extending first opening; wherein, the upper and lowerbeams and the second arm define a generally perpendicularly extendingsecond opening; and wherein, the two connection members respectivelyextend through the first and second openings to couple the pillow blockbetween the upper surface of the yoke and the hub plate.
 15. The hubassembly as recited in claim 1 wherein the four pillow blocks furthercomprise: first and second pillow blocks that are oppositely disposed onthe yoke, generally parallel to each other and define a first pillowblock axis extending generally perpendicularly therebetween; and thirdand fourth pillow blocks that are oppositely disposed on the yoke,generally parallel to each other and define a second pillow block axisextending generally perpendicular therebetween that is generallyorthogonal to the first pillow block axis.
 16. A proprotor system fortiltrotor aircraft having a helicopter flight mode and an airplaneflight mode, the proprotor system comprising: a mast; a yoke having fourblade arms and an upper surface; a plurality of proprotor blades eachcoupled to one of the blade arms of the yoke; a constant velocity jointassembly providing a torque path from the mast to the yoke, the torquepath including a trunnion assembly, four drive links and four pillowblocks, the trunnion assembly coupled to the mast and having fouroutwardly extending trunnions, each of the drive links having a leadingbearing coupled to one of the trunnions and a trailing bearing coupledto one of the pillow blocks and each pillow block independently mountedbetween the upper surface of the yoke and a hub plate with twoconnection members extending through the yoke and each pillow block, theconstant velocity joint assembly providing a gimballing degree offreedom for the yoke relative to the mast; and upper and lower hubsprings, the upper hub spring disposed between the trunnion assembly andthe hub plate, the lower hub spring disposed between the trunnionassembly and the yoke, the upper and lower hub springs operable todampen movement of the yoke in the gimballing degree of freedom;wherein, in the airplane flight mode, a first in-plane frequency of eachproprotor blade is greater than 2.0/rev.
 17. The proprotor system asrecited in claim 16 wherein, in the airplane flight mode, the firstin-plane frequency of each proprotor blade is less than 3.0/rev.
 18. Theproprotor system as recited in claim 16 wherein, in the airplane flightmode, the first in-plane frequency of each proprotor blade is betweenabout 2.2/rev and about 2.8/rev.
 19. The proprotor system as recited inclaim 16 wherein, in the airplane flight mode, the first in-planefrequency of each proprotor blade is between about 2.4/rev and about2.6/rev.
 20. The proprotor system as recited in claim 16 furthercomprising a leading edge pitch horn and a pitch control assembly havinga positive delta 3 angle coupled to each proprotor blade.